In many electronic applications, electrical resonators are used. For example, in many wireless communications devices, radio frequency (RF) and microwave frequency resonators are used as filters to improve reception and transmission of signals. Filters typically include inductors and capacitors, and more recently resonators.
As will be appreciated, it is desirable to reduce the size of components of electronic devices. Many known filter technologies present a barrier to overall system miniaturization. With the need to reduce component size, a class of resonators based on the piezoelectric effect has emerged. In piezoelectric-based resonators, acoustic resonant modes are generated in the piezoelectric material. These acoustic waves are converted into electrical waves for use in electrical applications.
One type of piezoelectric resonator is a Bulk Acoustic Wave (BAW) resonator. The BAW resonator includes an acoustic stack comprising, inter alia, a layer of piezoelectric material disposed between two electrodes. Acoustic waves achieve resonance across the acoustic stack, with the resonant frequency of the waves being determined by the materials in the acoustic stack. One type of BAW resonator comprises a piezoelectric film for the piezoelectric material. These resonators are often referred to as Film Bulk Acoustic Resonators (FBAR).
FBARs are similar in principle to bulk acoustic resonators such as quartz, but are scaled down to resonate at GHz frequencies. Because the FBARs have thicknesses on the order of microns and length and width dimensions of hundreds of microns, FBARs beneficially provide a comparatively compact alternative to certain known resonators.
FBARs may comprise an acoustic stack disposed over air. In such a structure, the acoustic stack can be referred to as a membrane. Often, the membrane is suspended over a cavity provided in a substrate. Other FBARs comprise the acoustic stack formed over an acoustic mirror formed in the substrate.
Filters based on FBAR technology provide a comparatively low in-band insertion loss due to the comparatively high quality (Q) factor of FBAR devices. FBAR-based filters are often employed in cellular or mobile telephones that can operate in multiple frequency bands. In such devices, it is important that a filter intended to pass one particular frequency band (“the passband”) should have a high level of attenuation at other nearby frequency bands which contain signals that should be rejected. Specifically, there may be one or more frequencies or frequency bands near the passband which contain signals at relatively high amplitudes that should be rejected by the filter. In such cases, it would be beneficial to be able to increase the filter's rejection characteristics at those particular frequencies or frequency bands, even if the rejection at other frequencies or frequency bands does not receive the same level of rejection.
One type of filter based on FBAR technology is known as a coupled resonator filter (CRF). A CRF comprises a coupling structure disposed between two vertically stacked FBARs. The CRF combines the acoustic action of the two FBARs and provides a bandpass filter transfer function. For a given acoustic stack, the CRF has two fundamental resonance modes, a symmetric mode and an asymmetric mode, of different frequencies. The degree of difference in the frequencies of the modes depends, inter alia, on the degree or strength of the coupling between the two FBARs of the CRF. If the degree of coupling between the two FBARs is too great (over-coupled), the passband is unacceptably wide, and an unacceptable ‘swag’ or ‘dip’ in the center of the passband results, as does an attendant unacceptably high insertion loss in the center of the passband. If the degree of coupling between the FBARs is too low (under-coupled), the passband of the CRF is too narrow.
The spreading of the passband due to overcoupling of the FBARs, and the swag in the center of the passband, has lead to efforts to reduce the degree of coupling between the FBARs of the CRF. For many known materials useful for acoustic coupling, the degree of coupling is too great, and results in an unacceptably high difference in the resonance frequencies of the modes of the CRF.
One known technique aimed at reducing the degree of coupling between the FBARs of the CRF involves the use of comparatively low acoustic impedance materials as the acoustic coupler. For example, silicon low-k (SiLK™) resin, which is known to one of ordinary skill in the art, has been investigated for use as an acoustic coupler in a CRF. While the use of known low acoustic impedance materials shows promise from the perspective of reduced coupling between FBARs in the CRF, and thereby improved passband characteristics, such known materials exhibit an unacceptably high acoustic attenuation resulting in an unacceptable degree of acoustic loss, and an undesirable reduction in Q.
For example, the acoustic attenuation at the even mode resonance (second resonance frequency) of a CRF having a SiLK™ coupling layer is unacceptably large, and causes a ‘tilt’ of the passband near the even mode resonance frequency. Furthermore, in many applications, CRFs are connected in series to provide multi-pole filters. Unfortunately, the unacceptable acoustic attenuation of SiLK™ at the even mode resonance frequency is additive in the series-connected resonators. The resultant insertion loss at the second resonance frequency and passband ‘tilt’ is thus further exacerbated.
Moreover, the step-coverage of spun-on SiLK™ in a CRF is non-uniform, and results in non-uniform coupling layer thickness, particularly near the perimeter of the acoustic resonator where topography is encountered. Such variations in the thickness of the layer of SiLK™ in CRF applications can perturb the coupling between the BAW resonators of the CRF. These perturbations can result in spurious modes and undesirable ‘notches’ in the passband of the CRF. Furthermore, SiLK™ has a comparatively large temperature coefficient of expansion and elastic stiffness. The resultant CRF has an unacceptably large temperature coefficient of frequency (TCF), which is not desirable in many applications.
What is needed, therefore, is a BAW resonator structure and method of fabrication that overcomes at least the known shortcomings described above.